Method of electrothermically smelting zinc



July 10, 1956 R. A. WILKINS ETAL 2,754,196

METHOD OF ELECTROTHERMICALLY SMELTING ZINC Filed April 12, 1951 4Sheets-Sheet 1 irroenzons a.m fizckard' vw 62., M, .W

' i WW @5- July 10, 1956 R. A. WILKINS ETAL 2,754,196

METHOD OF ELECTROTHERMICALLY SMELTING ZINC Filed April 12, 1951 4Sheets-Sheet 2 ,Ez/enZorfilackara a. Warns JfienrzeZia July 10, 1956 R.A. WILKINS EI'AL METHOD OF ELECTROTHERMICALLY SMELTING ZINC 4Sheets-Sheet 5 Filed April 12, 1951 July 10, 1956 R. A. WILKINS EI'ALMETHOD OF ELECTROTHERMICALLY SMELTING ZINC 4 Sheets-Sheet 4 Filed April12. 1951 fizz/0:201;

enne MW Wm fflw a Uflited States Patent 'fiFice 2 ,754,196 Patented July10, 1956 lVlETHOD 0F ELECTROTHERIVHCALLY SMELTIN G ZINC Richard A.Wilkins, Rome, N. Y., and Kenneth A. Phillips, Collinsville, 111.,assignors to Revere Copper and Brass Incorporated, Rome, N. Y., acorporation of Maryland Application April 12, 1951, Serial No. 220,574

7 Claims. (Cl. 75-14) relates to furnaces and apparatus comprising thesame capable of, among other uses, being used in the practice of suchmethods.

The invention has among its objects an increased recovery of zinc fromthe zinciferous material treated, the provision of a loose chargeenabling a continuous smelting operation to be readily elfected, and,when desired, the simultaneous recovery with the zinc of a mattecontaining the precious metal values of the zinciferous material. Theseand other objects of the invention will be best understood from thefollowing description of several forms of the method and apparatusaccording to the invention, while the scope of the invention will bemore particularly pointed out in the appended claims.

Commonly the significant zinciferous content of zinc ore as mined iszinc sulphide. After being mined the ore is crushed and subjected to aseparating operation for removing as much of the gangue as economicallyfeasible. The resulting zinc ore concentrate is then roasted in thepresence of air for converting the zinc sulphide to zinc oxide.Commercially the resulting product is commonly known as zinc sinter orzinc calcine, depending upon the exact roasting method employed, thesinter being more or less fused as distinguished from the calcine. Both,however, are granular materials or may readily be made such bysubjecting them to a slight crushing operation. Generically theseproducts are herein sometimes referred to as oxidized zinc ore inconformity with the terminology commonly employed in the smeltingindustry.

The oxidized zinciferous material produced by the above mentionedroasting operation contains some of the gangue of the original ore,which gangue is largely silica or other fusible substance capable ofslagging if heated to the requisite temperature, the melting point ofthe slag being about 1950 to 2250 F. depending upon its exactcomposition. Also this oxidized zinc ore contains other impurities amongwhich is practically always iron in the form of iron oxide and usuallysome iron in the form of iron sulphide, both of which may readily bereduced to produce metallic iron by heating the oxidized zinc ore in thepresence of carbon, such iron because of its carbon and silicon contentbeing liquid at the temperature of the liquid slag. In most instancesthe oxidized zinc ore further contains a small amount of copper in theform of sulphide, usually with some oxide or other copper bearingcompound or mineral which will form copper sulphide when the oxidizedzinc ore is heated in the presence of carbon and the residual zincsulphide in the zinc ore and sulphur in the coke or coal supplying thecarbon. Also the oxidized zinc ore commonly contains a small buteconomically valuable amount of the precious metals gold and silver.

According to the present invention, the oxidized zinc ore containing thezinc oxide and impurities is mixed in granular form with reaction carbonin granular form and the mixture charged to a hearth in an operativelyclosed chamber to form on the hearth a mass of the charge having anupper downwardly sloping free surface which at its lowermost portion issubstantially in the plane of the hearth. This reaction carbonpreferably is in the form of coke or anthracite coal rendered granularby a crushing or grinding operation, or coke breeze, or bituminous coalfines. In the particular forms of apparatus illustrated by the drawingsthe mixture is fed to the chamber from below the sloping surface of themass of charge and adjacent its higher portion so that the charge beingfed forces the material of the mass to this sloping surfacesubstantially throughout its entire extent and thus also acts to moveaway from the body of said mass the material of the mass adjacent thelower edge of said sloping surface, the slant of this sloping surfacebeing largely determined by the angle of repose of the granular charge.Feeding the material to the mass in this way causes it to be preheatedwhen it reaches the sloping surface of the mass as hereinafter morefully explained, and thus avoids any necessity of preheating the chargeexteriorly of the chamber preliminarily to entering it thereinto aswould be the case if the charge were fed to the chamber from above saidsloping surface and it were desired to enter it in preheated condition.Heat is radiated downward on said sloping surface to heat the surfaceportion of the mass of charge to a temperature which will react the zincoxide with the carbon and melt the slag forming materials of theoxidized zinc ore and of the coke or coal of the charge. This heat alsois sufiicient to react with the carbon at least some of the iron oxidepresent, and some of any iron sulphide present, at the surface portionof the mass so as to produce molten iron as well as molten slag, thecopper sulphide dissolving into the iron so that the iron significantlyis in the form of an iron-copper matte mixture. This iron also containsdissolved carbon and commonly silicides which act to reduce its meltingpoint. Such molten slag and iron so produced adhere as more or lessviscous droplets to solid particles of excess coke or coal and to othersolid particles which may be present in the residue. Any of such liquidslag or iron which tends to percolate downward into the mass will adhereas droplets to the solid particles of said mass relatively close to itssurface portion, and, because of the lower temperature within the mass,may even solidify. However, such slag and iron so percolating will bereturned to the surface portion of the mass by the incomingcharge andthere be remelted. The zinc produced is in the form of a vapor which,together with the carbon monoxide and other gases formed by the reactionand heat, escapes upward from the free surface portion of the mixture.These escaping vapors and gases agitate the upper surface portion of themixture and cause the excess coke, coal and other solid particles, withthe slag and iron droplets adhering to them, to gravitate down thesloping surface to its lower edge portion for removal from the chamber.The zinc vapors and gases which escape upward from the mass may bedischarged from the chamber into a condenser where they are cooled forcondensing the vaporous zinc to liquid zinc.

In the above described operation the residue which reaches the loweredge portion of the sloping surface of the mass of charge, because itconsists in effect of molten slag and iron mixed with a variable amountof solid particles to which the molten constituents adhere, is ratherpasty or sticky and does not flow readily. This residual materialhowever may be readily removed from the furnace, if not too much solidmaterial is present in it, by causing it to be heated by the downwardlyradiated heat, when it reaches the lower edge portion of the slopingsur- 3 face. and wh n t Pu he by h e din m ntion cuts a portion of thehearth beyond the mass of charge, to render it sufficiently liquid orlimpid to be tapped from the furnace.

The molten iron-copper matte mixture, as itis formed at the slopingsurface portion of the mass of charge and, particularly as it isrendered limpid on. the portion ofthe hearth beyond the mass of charge,acts to scavenge the residue of any precious metal values present in theoxi ized zinc ore charged into the furnace, such values going intosolution in such mixture. After the ironcopper matte mixture and slagwith the. solid particles of the residue are tapped from the furnace theiron-copper mattem-ixturecan readily be recovered by liquation andtreated for recovery of those precious metals by any one ofseveralmethods used by those familiar with the art.

The oxidized zinc ore seldom contains less than about iron in the formof iron oxide usually with a small amount of iron in the form of ironsulphide. The total amount of iron in these forms varies considerablyand commonly runs as high as 14%. Sufficient iron oxide,

or iron sulphide or other ironcompound capable ofbeing reduced tometallic iron by the carbon, will'be contained in the charge in amountsufficient to produce enough metallic iron to take up sulficicnt coppersulphide satisfactorily to scavenge 1e charge of its precious metalvalues if the amount of iron chemically so contained in the oxidizedzincore is about 1%. in the rare case of an oxidized zinc ore which does notcontain enough iron oxide or other carbon reducible iron compound thedeficiency may be corrected by adding a requisite amount of the same tothe charge with the reaction carbon, or even adding chips of cast ironof high enough carbon content to be readily ineltaole.

Copper sulphide dissolved in the iron acts very markedly to augment itsproperty of scavenging the charge of its precious metal values. Thecopper sulphide, it has been found, will give appreciable results inthis respect when the copper contained therein is chemically at leastabout 0.1% of the oxidized zinc ore. Ordinarily the ore will contain atleast that amount of copper in the form of copper sulphide. in the rarecase of a zinc ore which is deficient in copper sulphide such deficiencycan be corrected by adding it, or copper in the form of any of the othercompounds or minerals above mentioned, to the charge when the coke orcoal is added, or even by adding metallic copper which will be convertedto copper sulphide by theresidual zinc sulphide contained in theoxidized zinc ore and any sulphur contained-in the coke or coal.

The particular size of the granular coke or other carbonaceous reactionmaterial employed and of the oxidized zinc ore is by no means critical,satisfactory results being secured if the particles of maximum size willpass a one inch mesh screen, the bulk of the particles of'course beingmuch smaller down to what would constitute powdered reaction coke orcoal and oxidized zinc ore. However, unless the oxidized zinc ore isporousin structure the rate of reaction willbc substantially increasedby crushing it somewhat more finely.

The temperature to which the surface portion of the charge is heated ofcourse depends uponthe amount of heat radiated downward upon it. From acommercial standpoint, significant amounts of zinc vapors will beproduced when the surface portion ofthe charge is heated to about 1700F. It has been fpund, however, that the maximum temperature to which thesurface portion of-thecharge can be heated is in the neighborhood of23O0*F any increased amount of heat radiated down; ward on the chargemerely serving to speed up th e.reac tion, theelfect being analogous tothat observedlin boiling water which may be heated 'to a boiling pointof definite temperature without-any excess amountof heat raising itstemperature. but merely increasing the rapidity with which the. .water.boils to-formsteam.-

Accprding to the presentinvention, preferably sufiicient' xcess heat. iresisted tower ai on. he. charge he its upper free surface portion tothis maximum temperature and cause the reaction to take place rapidly soas to secure in effect a flash reaction. The molten slag and iron willbe produced at about 1 950 to 2250 F., as likewise will be produced themolten copper sulphide which dissolves into the iron, and consequentlythese molten materials will be produced: at the maximum temperaturementioned to which the surface portion of the charge is heated;

Because of the reaction being endothermic and the charge being fedsubstantially continuously tothehearth at the same rate as the charge isconsumed by the reaction, and said charge-being of low heat conductiyitthe reaction is confined substantially to the sloping surface portion ofthe charge and the molten droplets fail to penetrate the charge to anysubstantial extent below said surface portion without solidifying, withthe result thatctbe charge remains loose and isreadily substantiallycontinuously, forced; to said sloping surface portion as it is fed.

to the hearth. Furthermorqas the melting points ofthese droplets arelessthan the temperature at which the reaction will occcur, anydropletswhich tend to penetrate downward into the charge tend to solidify withinthe zone inwhich zinc vapors and gaseous reaction productsare evolved,which vapors and products tend to agitat thecharge as they. escapeupward therefrom and hence tend to move the dropletstoward the hightemperature surface of the charge where if. they have solidified theyare remelted.

It will be observed that by entering the charge into the furnace chamberat a point below the upper surface portion of the charge therein andheating said surface portion to a temperature well above the temperatureat which the reaction would begin to occur results in agradualpreheating of the charge asit-movesfrom its point of entry intothe chamber toward said surface portion, so

that the charge when it reaches said surface portion in condition tofacilitate the above explained rapid or flash reaction occurring at saidsurface portion.

The amount ofcoke or coal mixed withthe oxidized zinc ore charged to thereactionchamber, if the residue is to be removed fromthat chamber as aliquid containing little solid residue, may be from 5 to 30% in excessof that which will give the amount of carbon stoichiometricallynecessaryto reduce all zinc oxide and the-necessary amount of iron oxide or othercarbon reducible ironcompound present. Ten per cent excess carbon inrespect to that stoichiometrically necessary tore-:luce the zinc oxide,it-has becn found, ordinarily will give excellent results in regard bothto reducing the zinc oxide and producing the necessary amount of iron.With these amounts of excess coke or coal the pasty residuecollecting-at thelower end of the upper sloping surface of the charge onthe hearth may be rendered sufficicntly liquid with the molten slag oriron in the form of droplets adhermg; to particles of coke or coal thatit becomes increas inglydiflicult to render the'residue sufficientlyliquid or limpid-totap it from the reaction chamber, so that withsuch-excess amount of coke or coal it becomes necessary to rake theresulting pasty massof particles f rom .the chamber. In some cases.however, it may be desirable If the amount of excess colte or coal"is'much increased beyond the percentages thereof indh cated so'rnuchsolid coke or coal particles willjbernixed not to render the residueliquid enough to tap it from the chamber but to rake it therefrom,especially if the oxidized zinc concentrate does not contain preciousmetal values to be recovered, and in such case about 80 to 125% excesscoke or coal is preferably employed.

The apparatus illustrated by the accompanying drawings may be employedfor practising the above described methods, the drawings showing severalmodifications of electric furnaces which may form part of saidapparatus.

In the drawings:

Fig. 1 is a central longitudinal vertical section, on the line 11 ofFig. 4, of a furnace for practising the methods according to theinvention;

Fig. 2 is a section on the line 2--2 of Fig. 1, this section line forconvenience in following the drawings also being applied to Fig. 4;

Fig. 3 is a section on the line 33 of Fig. 2, with parts omitted;

Fig. 4 is a more or less diagrammatic plan view of apparatus forpractising the methods according to the invention, with parts omitted;

Fig. 5 illustrates more or less schematically a modified form offurnace; and

Fig. 6 illustrates a fragment of a further modified form of furnace.

Referring to Figs. 1, 2, 3 and 4 of the drawings, the furnace comprisesan open top casing 1 of sheet metal, preferably steel, having side walls3, 5, 7 and 9 and a bottom wall 11 all welded together to form a fluidtight construction. As shown, this casing is provided with a cover 13comprising a reinforced metal plate 15 the bottom surface of which restson the upper edge of the side walls of the casing. The cover isremovably secured to the casing by the bolts 17 welded at their lowerend portions to brackets 19 (Fig. 2) secured to the stiffening bars 21for the side walls 7 and 9 of the casing, to which latter the inneredges of these bars are welded. Extending continuously around the upperedge portions of the side walls of the casing 1 are angle-irons 23 theedges of the horizontal webs of which are Welded to the casing sidewalls in a fluid tight manner to form a trough 25 which is filled withsand, oil or the like and into which projects a flange 27 extendingcontinuously around the edge of the cover plate 15 at its under side influid tight welded relation thereto. In this way the cover is secured tothe casing in a fluid tight manner.

Internally the casing 1 contains a structure which forms a furnacechamber having an upper portion 29 and a lower portion 31. The bottom ofthe lower portion 31 is built up of hard carbon blocks 33 to form ahearth 35, which hearth, as shown by Fig. 2, is of arcuatecross-section. The blocks 33, as shown by Fig. I, extend from one end ofthe furnace chamber to a point spaced from its opposite end to form awell or pit 37 having a bottom wall 39 formed by the hard carbon blocks41.

At opposite longitudinal sides of the hearth the blocks 33, as shown byFig. 2, abut the wedge-shaped blocks 43 a row or" which extends for theentire length of the chamber. On the upper flat surfaces of thesewedgeshaped blocks are supported at opposite sides of the chamber theelongated horizontal graphite slabs 45, similar slabs of graphite 47being positioned at opposite ends of the chamber. These slabs 45 and 47are grooved at their upper sides as shown at 49, these groovescommunicating with each other at the four corners of the chamber. Thegrooves in the slabs 45 receive the lower edge portions of verticallypositioned horizontally extending graphite slabs 51, and the grooves inthe slabs 47 receive the lower edge portions of vertically positionedhorizontally extending graphite slabs 53, the slabs 51 and 53 abuttingat the four corners of the chamber. Supported by each of the slabs 51 attheir upper edges are graphite bars 55 which, as shown by Fig. 4, extendfor the length of the chamber, while supported by each .of the slabs 53at their upper edges are graphite bars 57 which extend for thetransverse width of the chamber, the bars 55 and 57 abutting adjacentthe four corners of the chamber and being grooved at their lower sides,as shown at 59, for receiving the upper edges of the slabs 51 and 53.Resting on the upper sides of the bars 55 are vertically positionedhorizontally extending graphite slabs 61, while resting on the bars 57are vertically positioned horizontally extending graphite slabs 63,these slabs 61 and 63 abutting each other at the four corners of thechamber, and the upper sides of the bars being grooved, as shown at 65,for receiving the lower edges of the slabs. The roof of the chamber isformed by the laterally abutting elongated graphite slabs 67 which restat their opposite end portions on the top edges of the slabs 61, theedge portions of the slabs 67 adjacent the slabs 63 also resting on thetop edges of the latter.

The carbon blocks 33 and 41 forming the hearth 35 and the bottom wall 39of the well or pit 37, respectively, are supported on a layer formed bycourses of firebricks 69, which layer at the sides of the wedge-shapedblocks 43 and adjacent the longitudinal ends of the casing 1 is bu ltupward to support and back the slabs 45 and 47 and to form firebricklayers 71 adjacent the side walls of the casing. On the tops of thefirebrick layers 71 rest the elongated graphite slabs 73 which close thespace surrounded by said layers. The space between these slabs and thefurnace chamber roof slabs 67, and the space between the firebricklayers 71 and the slabs and bars forming the interior side walls of saidchamber, are filled with a mass of heat insulating material 75 as, forexample, carbon beads or a mixture of the same and lamp black or brokenup charcoal.

Between the slabs 73 and the cover 13 of the casing 1, and between thatcasing and the firebrick layers adjacent its sides and bottom, areinserted heat insulating packing layers 76 of material capable ofresiliently yielding under high pressure to permit the furnace casingand the interior walls of the furnace to expand and contractindependently of each other was to prevent rupture of the casing.

In the upper portion 29 of the furnace chamber is positioned a row ofparallel elongated heating resistor bars 77 preferably of graphite. Theopposite end bars of the row have extensions 79 projecting through thefurnace walls in electrically insulated relation thereto, and at theexterior of the furnace these two extensions are provided with terminals81 for the cables for energizing the resistors, the resistors beingconnected at opposite ends for series flow of current through them. Theresistors are supported from the roof slabs 67 by the elongated graphitebars 83, which latter are carried by said slabs in electricallyinsulated relation thereto. The construction of the resistors and theirsuspension is more fully described in Poland United States Patent2,472,613, issued June 7, 1949, and in pending United States applicationof Richard A. Wilkins, Serial Number 162,220, filed May 16, 1950, andneed not be further described here. The resistors when heated becomeincandescent and radiate beat downward toward the hearth. They also actto heat the graphite roof of the chamber to iucandescence to cause italso to radiate heat downward toward the hearth.

As shown, interposed between the resistors and the hearth is a row oflaterally abutting elongated bars 85, of circular cross-section, formedof graphite which is extremely heat refractory and an excellentconductor of heat. These bars form a partition dividing the furnacechamber into its upper and lower portions 29 and 31. They serve toprotect the resistors from attack by dust, vapors and gases emanatingfrom the heated charge. The circular cross-section of the bars gives theupper and lower sides of this partition a corrugated shape whichincreases the area of those sides and therefore increases the facilitywith which the partition absorbs the heat from above and radiates suchheat downward toward the hearth.

As shown, the bars 55 are formed to provide shelves 37 on which the endsof the bars 85 rest, these shelves 7 being inclined downward from onebar 57 t the other so that when the bars areplaced on the higher ends ofthe shelves they will roll downward to the lower ends of the shelves andthere rest against the abutments 39 formed by the bars 57 at said lowerends. The bars 35 may be inserted and removed from the furnace withoutcooling it. For this purpose the walls of the furnace are provided withopenings in one which adjacent the higher end of the inclined shelves 87is positioned a graphite sleeve 91 (Fig. 4), and in another of whichadjacent the lower end of those shelves is positioned a graphite sleevesleeves are received at their inner ends in the enlarged diameterportions of openings '97 in the adjacent bar 55. The sleeves arenormally plugged with removable stoppers in the form of bars 99preferably of hard carbon which is a relatively poor heat conductor,these has being preferably of such length as to extend for the fulllength of the sleevesso as to minimize heat lossesfrorn the chamber. Asshownjthose ends of the sleeves M and )3 which project from the outerside of the furnace are surrounded by the water jacketed metal sleeves143 i welded at one end to the casing 1, which sleeves 19h keep theouter ends of the carbon bars 99 cool enoughto he conveniently handled.Upon removal of the plug from the sleeve 93 the bar 35 at the lowermostends of the inclined shelves 87 may be pushed out of the furnace'through such sleeve by means of a metal rod inserted and pushed throughthe graphite sleeve fill of small diameter bore positioned in an openingin the furnace wall opposite the sleeve 93 in alignment therewith, thissleeve 191 communicating with an opening 1% formed in the adjacent bar55. The bore of the sleeve 191 may be normally plugged with a stopper inthe form of a-carbonbar similar'to one of the bars 99. Ordinarilyhowever, due to the small diameter of the bore of the sleeve ltil, itwill suffice if such bore is closed by a metal cap screw-threaded on'orotherwise removably carried by a'rne'tal sleeve M57 which surrounds theoutwardiy projecting end of the sleeve loi and is Welded to the adjacentwall of the casing 1. For supporting the bars 85, when they are beinginserted and removed from the furnace chamber, the bars 57 are eachformed with a shelf 14?? on which the bars slide for preventing themfrom tilting and fallingdownward into the lowerporticn of the furnacechamber. it will be understood that upon removal of abar 85 at the lowerends of the inclined shelves 8? the rniain'ingbars will roll down saidshelves to place the next consecutive bar 35 opposite the sleeve 93 andthus leave a space at the higher ends of such shelves into which a newbar maybe inserted through the sleeve '91 whenthe-stopper bar99 in thatsleeve is removed.

As further shown, the furnace according to Figs. 1, 2, 3 and 4 isforrnedwith a tap hole-111 communicating with the'w'ell'orpit 37, this tap holebeing normaliyclosed by a removable plug 113 of fireclay or the like,upon reinov'alof which the material in the well may be tapped therefromas a liquid mass or be raked therefrom depending, upon the temperatureof such mass and the arnount of solid'p'articles contained "therein.

Also, preferably, 'the exterior of the casing 1 at its lower portion is"surrounded by a water jacket 115 which has the effect of 'c'ooling theexposed surfaces of the hearth andwell 37 so that tnere'will beforr'nedthereon a rather 'thin layer of solid iron or slag or a mixture of thetwo, the iron usually being in th'eiorrn of magnetite. This layerprotects "the carbon blocks forming those surfaces'fr'om destructionby'the niolten'irom'which tends 'to'diss-olv'e them, and by theresidual'zinc oxide dissolved or mixed in the slag, which zincofridetends aggressively to react withthem. The iron and slag have about thesame'melting point, alth'oughthe same may vary 7 somewhat with eachdepe'ndin'g iipo'n its'eXact composition. The cooling'efiect'of thewater jacket is so "cobrdinated with the'thickness and heat conductivityof the materials forming 'the"adjace'nt portion of the furnace 7 Wallsthatwhen molten slag or iron contacts the surfaces ofthe hearth'and Wellit will solidify and its thickness increase untilits surface "temerature reaches the melting point of the'sla'g or iron, whereupon the'pr'o- I tec-tive layer thus formed "will not build up farther, or, ifthe slag or iron when it 'firs'tcontac'ts these surfaces of the hearthand well eats them away, the consequent slight thinning of the blockspresenting those surfaces will cause such surfaces to reach atemperature which will solidify the slag or iron contacting them, whichsolidified slag or iron will build up to form a protective 7 layer untilthe thickness of such layer is such that its exposed surfacesare attheir'm'eltin'g points.

In the above connections it will be understood that in operationnormally only the well 37 and the portion of the hearth at th'e'le'ft ofthe charge as viewed in Fig. l, and its portion below the chargeadjacent the lower end of the upper sloping surface of the'charge, willbe subjected to the form of destruction mentioned as its portions wellbeneath the'cha'r'ge will, because of the heat insulating efiect'of thecharge, be ordinarily cool enough to prevent liquid slag or iron fromreaching them. However, the hearth throughout its entire length willtend to be subjected to such destruction when initiating the chargingoperation with a preheated furnace, and consequently the water jacketpreferably surrounds the entire lower 'p'ortion'of the furnace.

As shown, the vertical wall of the furnace adjacent the end of thehearth opposite the well 37 comprises a sleeve 117 of hardcarbon'havi'ng a tapered bore 119. At its outer end this bore isprovided with a metal sleeve extension 121 communicating with thehorizontal con duit 123 (Figs; 1 and 4) at the exterior of the furnace.This conduit communicates at one end with the bottom discharge opening125 (Fig. 4) of a hopper 1247, which opening is provided with a gatevalve 129, shown in partly closed position in Fig. 4, so that theconduit 123 can be closed when desired. Extending lengthwise of theinterior of the conduit is an elongated rotary screw conveyor 131schematically shown as driven by an electric motor 1-33 througha'reducti'on gear (not shown) contained in a casing 135. The mixture ofoxidized zinc oreand'coke or coal 'is entered into the hopper so as todischarge into the conduit 123'through which the screw conveyor forcesthe material and causes it to pass through the bore 1' 19"t'o"feedsu'chmaterial to the furnace and form on the hearth a pile orm'ass 137 '(Fig.1') having a downwardly 'slopingrree upper surface 139 theslo'pe ofwhich is determined by the angle of repose of the material. Thismaterial builds up'at its higher end to above the bore 119, and "theinc'or'ning material from said bore .acts to force the material toward thesloping surface of the pile substantially throughoutthe entire extent ofsaid surface and to 'move the residue at the lower end a of thepile'along'the"hearth in a'dire'c'tionwhich is away from said bore. 7

The heat radiate'd'downwardon the upper slcping sur face of the mass ofcharge will cause, in the way hereinbefore described, solid particles ofthe excess coke or coal and other residue with adhering dropletsof'molten slag and iron to gravitate down the sloping surface of themass of charge to the lower edgeportion of said mass, while the incomingcharge will cause such particles with the "adhering droplets to bepushed on the portion of'thehearth beyond the lower edge of said slopingsurface. On 'suc-h'portion of "the hearth, if t-here'is not too'muchexcess of coke or'coal and other solid re'si'= due, the heatradiated downward on that portionlwill render this slag or ironlsu'fficiently liquid or limpid to flow into 'the'well or pit 37 andcarry with it such excess coke or coaland other solid residue, fromwhich well the nrater'iallrnaybe tapped through the tap hole 111, ashereinbefore described; If there is a largeex- 'cess of coke 'or coalthe same withthe adhering 's'lag 'or iron droplets will buildup on theportion 'of the hearth just mentionedan'd work toward "the edge of thehearth arcades 9 and fall into the well as a sticky or pasty mass whichcan be raked from the well through the tap hole.

As shown, the hearth is provided with a bore 141 (Fig. 1) leading to theexterior of the furnace. In this bore is removably inserted a tube 143having a thermocouple at its outer end 145, leads 147 from thethermocouple extending through the tube to the exterior of the furnacebeing connected to a suitable instrument (not shown) for indicating thetemperature of the thermocouple. For a given amount of heat radiateddownward on the charge, and a given thickness of the portion of thecharge above the thermocouple, the thermocouple will indicate a definitetemperature, this temperature being normally somewhere about 1900 F. ifthe thermocouple is, as shown by Fig. 1, positioned beneath the chargenear the lower end of its upper sloping surface and sufficient heat isradiated on the charge to secure the flash reaction hereinbeforedescribed. The thermocouple may, if desired, be placed under a thickerportion of the charge where because of the heat insulating effect of thecharge the temperature of the hearth will be lower, but best results ithas been found will be secured by placing it under the charge near thelower end of the upper sloping surface of the charge. By observing thetemperature indicated by the thermocouple when satisfactory operation issecured, and maintaining a rate of feed of the charge to the furnace tokeep that observed temperature approximately constant, such satisfactoryoperation will be continued if the amount of current flowing through theresistors is kept constant.

The vaporous zinc admixed with the gaseous products of the reactionescape from the furnace chamber through an opening 149 in the furnacewall, from which opening they are led through a conduit 151 to acondenser 153 (Fig. 4) where the vaporous zinc is condensed to liquidzinc, the gaseous products escaping from the top of the condenserthrough a stack 155 provided with a damper 157 which may be adjusted toregulate the pressure within the condenser and furnace chamber. Normallysatisfactory results will be secured with a pressure in the furnacechamber and condenser just slightly above atmospheric. So maintainingthe pressure prevents leakage of air into the furnace chamber andcondenser through any possible interstices in their walls as well asthrough the charging opening 119. It also prevents air from entering thefurnace chamber when the tap hole 111 is opened for discharging liquidresidue from the well or pit 37 or for raking sticky or pasty residuetherefrom. Conveniently, when such residue is to be removed from thewell, the energy input to the resistors is temporarily reduced todecrease the evolution of zinc vapors from the charge so as to decreasethe amount of the same that might escape from the furnace chamberthrough the tap hole when it is opened, the damper 157 being adjusted,if necessary, when the tap hole is opened to insure that the pressurewithin the furnace chamber remains siightly above atmospheric.

Maintaining the mass of charge on the hearth adjacent the bore 119 wellabove that bore, as shown in Fig. 1, acts to insulate the material inthat bore and that portion of the material on the hearth which isadjacent the discharge end of said bore from the downwardly radiatedheat so that no substantial reaction between the zinc oxide and carbonwill occur in such material so insulated. If it did occur the zincliberated would tend to condense at the cooler portions of the bore andtend to clog it. For further insuring that no reaction of the zinc oxideand carbon will occur in the bore 119 the portion of the water jacket115 at the end of the furnace adjacent that bore is extended upward toform a portion 158 that surrounds the sleeve extension 121 of the bore,and by absorbing heat from the adjacent portion of the furnace wallsacts to keep the material in said bore and extension at a temperaturebelow that at which any substantial reaction between the zinc oxide andcarbon oc curs. For preventing zinc vapors from being forced from thefurnace chamber through the charge into the bore 119 and chargingconduit 123, and into said bore and conduit when the charge is beinginitially entered into the preheated furnace or when the mass of chargeon the hearth is otherwise at a low level, in which bore and conduitsuch vapors would condense to troublesome solid metallic zinc andinterfere with the charging operation, the block or sleeve 117 is shownas formed with a passage 159 (Fig. l) which opens into the bore 119 ofsaid block. Through this passage an inert gas, such as nitrogen, may befed to the bore 119 from a supply pipe 161 communicating with saidpassage, the flow of nitrogen being controlled by a valve 163 in saidpipe. The amount of nitrogen so admitted to the bore 119 may beregulated by the valve to cause the pressure in that bore to be slightlyhigher than that in the furnace chamher so as to prevent backflow of thezinc vapors into said bore and the conduit 123. Nitrogen from thispassage may also be admitted to the furnace chamber and hence to thecondenser for displacing the air contained in them prior to heating theresistors and furnace chamber preliminary to initiating the smeltingoperation.

If desired, the furnace chamber may be fed with charge from each ofopposite ends of the hearth as schematically illustrated in Fig. 5, inwhich case the well or pit 37 may be positioned midway the length of thehearth, two sets of resistor bars '77, and bars for protecting saidresistors, preferably being provided in such case.

In the practice of the method as hereinbefore described the residue maybe rendered sufiiciently liquid or limpid to tap it from the furnace ifthe excess amount of coke or coal is small, whereas if such excess islarge the residue will not be so rendered and must be raked or the likefrom the furnace in the form of a more or less sticky mass ofsubstantially solid particles. However, by use of the furnace accordingto Fig. 6, the residue in its last mentioned form may be raked from thefurnace when employing the smaller amounts of excess coke or coal, itbeing understood that it may be of advantage to perform the method inthis way when the oxidized zinc ore does not contain precious metalvalues or it is not economically feasible to recover them. Employing thesmaller amount of excess coke or coal of course effects a considerablesaving in these materials, while not heating the residue sufiiciently torender it flowable conserves heat energy.

The furnace according to Fig. 6 is identical with that shown by Figs. 1,2, 3 and 4 except that the well or pit 165 of that according to Fig. 6is deeper and Wider than the well or pit 37 of that according to Figs.1, 2, 3 and 4. Above the well 165 the adjacent side wall of the furnaceis provided with elongated carbon blocks 167, 169 and 171 which projectfrom said well to form a shield above the well and the adjacent extremeend portion of the hearth 35 to shade the Well and that portion of thehearth from the downwardly radiated heat. This shading of the well andadjacent portion of the hearth insures that the residual material in thewell and on that portion of the hearth will not be heated to a highenough temperature to make the slag and iron sufiiciently liquid orlimpid to permit the residue to be tapped from the well, the residue inthis case being raked from time to time from the well through thenormally plugged hole 111 in the form of a more or less sticky mass ofsolid particles. To enable the residue to be raked from the Well or pit165', when using the furnace according to Fig. 6, the mixture ofoxidized zinc ore and coke or coal is fed to the furnace chamber at suchrate as to maintain on the hearth a mass having an upper downwardlysloping surface 173 the lower edge 175 of which is at the portion of thehearth shaded by the above described heat shield so that the residue onthe hearth and in the mass of charge adjacent the edge 175 will not beheated sufliciently to make it liquid or limpid and will be pushed ofifthe hearth and fall into the well or pit 165 in the form of the abovementioned more or less sticky'mass of solid particles. The material inthe well, being effectively shielded from the downwardly radiated heat,in some instances will further cool to substantially solidify the liquidor pastry droplets of slag and iron adhering to solid particles of theresidue so that adjacent the hole "111 as the residue accumulates in thewell such residue'will be more or less in the form of a dry mass ofgranular particles.

in all instances of the practices of the methods described the chargemay be fed to the furnace chamber substantially -continuously as anuninterrupted stream, or as an intermittently flowing stream, or inbatches each "constituting a relatively small fraction of the charge onthe hearth, so long as the overall rate of feed is such thatitsubstantially'cqualstheoverall rate at which the charge is consun'ie'dby the'reaction and a mass of charge having an upper-sloping surface anda portion above the level of the charging'port is maintained on thehearth during the continuance of'the smelting operation. It will also beunderstood that specific feeding means herein disclosed constitutes'insubstance a form of under feed stoker and that other suitable formsof under feed stokers may be substituted for it.

@ther oxidized zinciferous substances have compositions analogous tothat-of the roasted or oxidized zinc sulphide ore concentrates abovedescribed in that they contain zinc oxide, iron or carbon reducible ironcompound and slagforming'materials, and, in some instances, "coppersulphide or mineral 01' compound which in the presence of zinc sulphideor sulphur in the charge will cause copper sulphide to be formed, andalso, in some instances, precious metal values. Among these arenaturally occurring oxidized zincores and concentrates of the same,so-called -densitied zinc oxide, the residue of conventional'zincretorts, galvanizing dross, the waste products of lead smelters, certainzinc dusts, and other -zinciferous residues, drosses, slags, dusts,wastes and secondary-materials. These other oxidized zinciferoussubstances =may be treated according to the "present inven- "tion, inthe same way as the roasted or oxidized zinc sulphide ore concentratebycrushing them to render them granular when they are not already insuitable powdered or otherwise granular form, and mixing them withgranular reaction carbon when 'they do not already contain the same orcontain'an insuflicient amount thereof, sufficient 'cop'persulphide'orsubstance that will form copper sul phide in the way hereinbeforedescribed also being adds-iii necessary, to enable the precious metalvalues tofbe recovered if suclrvalues are-present and it is desiredwhichdo notin themselves contain slag forming material iron may alsobe'treated according to the invention byrend'ering them granular bycrushing them, or, if in 'i' cry finely divided form, by balling orbriquetting them and, it necessary,-crushing the balled or briquettedma- 7 teri'al to secure-the desired granule size, after which thegranular material is mixed with granular reaction carbon to form thechnr e, iron or carbon'rcducible iron compo'und "and copper sulphide-orthe likebeing added'only' ifdhc' matcrial contains precious metal valuesthat it is "desiredt'o recover.

For convenience in terminology, in the appended oxidized zinc sulphideore concentrate and d 'ziuciferous substances above mentioned causes and7 action carbon in'substance to contain copper sulphide) 1 are referred'togenerically as copper sulphide.

litthe a'bove c'onnections it will be understood that theprese'nceof-slag forming materials in the charge is not essentialtotherecovery of the'zinc, or to the removal to recover them. Other oxidizedzinciferous materials of the residue from the furnace chamber unless itis desired to remove it therefrom as a liquid containing the solidparticles, although as a practical matter slag forming materials alwayswill be present if coke or coal of usual composition is employed topresent the necessary reaction carbon, and, of course, iron and coppersulphide are not necessary if the zinciferous material contains noprecious metal values or it is not desired to recover them. in any rarecase where it may appear 'scessary slag forming materialsma-y he addedto the charge to enable the residue to be tapped in such liquid form.

it will be also understood that within the scope of the appended claimswide deviations may be made from the forms of methods, apparatus andfurnaces described, without departing from the spirit of the invention.

We claim:

l. The method of smelting zinciferous material containing zinc oxide andslag forming constituent which comprises feeding a granular mixture ofsuch material and reaction-carbon to 'a hearth in an operatively closedchamber at such rate as "to maintain continuously on said hearth-duringthe-continuanceof the smelting operati'on a mass having an upper surfacesloping downward to adjacent an edge portion of said hearth; radiatingheat downward from above said mass, while shielding the portion thereofadjacent saidedge portion of the hearth from such heat, to react thezinc oxide and the carbon at the upper surface portion of the unshieldedpart of said mass to liberate vaporous zinc from such'portion and meltslag forming constituent of said surface portion to cause molten slagadmixed with solid particles of the residue to gravitate down saidsloping surface to the shielded part of said mass; removing the slag andresidue mixture from said hearth in non-liquid condition, and

discharging the vaporous zinc from said chamber;

' 2. The method according to claim 1 in which the mixture is fed to themass of charge on the hearth from beneath the sloping surface of saidmass adiacent the higher portion of said surface.

3. The method of 'electrothermically smelting zinc from a mixturecontaining zinc oxide, excess reaction carbon, and slag formingsubstance, which comprisesfeeding such mixture in granular form'to'ahearth in an operatively closed chamber at such rate as to maintaincontinuously on such hearth during'the continuance of the smeltingoperation a mass having an'upper surface which slopes downward tosaidhearth at anedge portion of said mass at an inclination determinedsubstantially by the angle of repose of such granular mixture;continuously heating the sloping upper surfaceportion of said massduring the COHIlIlUEIHCQ'Of'il'lG smelting operation-by .eat caused'tobe radiated uponitby electric heating resistors positioned above saidhearth -and mass to react at said surface portion zinc oxide and carbonof'said mixture forforming vaporous zinc and gaseous reaction products,the latter consisting mostly ot'carbon monoxide, and for forming slagwhich adheres'to solid'particles of the residue of the mixture at saidsurface portion; said vaporous zinc and gaseous reaction productsfilling said chamber and being discharged therefrom for condensation ofthe vaporous zinc; the heat radiateddownward on such surface portionbeing of'such intensity as *to'cause said vaporous zinc and'gaseous'rcaction products while-escaping from saidsurface portiontoiagitate it tocause such solid residue and adhering slagtogravitatcidown said sloping surface portion to the portion of thehearth at the lower edge portion 'of said surfaceportion where it isreceived by the hearth contact with said mass of mixture onthe hearth;and from.

time to time as said slag accumulates in said heated por- 13 tion of thechamber removing it and such of said residue as remains with it fromsaid chamber.

4. The method according to claim 3 in which the granular mixture is fedto the mass of mixture on the hearth at such position below the slopingsurface portion of said mass as to force the mixture toward said surfaceportion substantially throughout the extent of said surface portion forcausing the incoming mixture to reach said surface portion in preheatedcondition, and for causing the slag and residue gravitating down saidsurface portion to the hearth to be pushed over said hearth away fromsaid mass.

5. The method according to claim 3 in which the heat radiated toward thehearth at the portion thereof which receives the pasty mixture ofresidue and slag that gravitates down the sloping surface portion of thecharge on the hearth is suflicient to render that mixture sufiicientlylimpid while on the hearth to cause it to flow to that heated portion ofthe chamber which is below the level of the adjacent portion of saidcharge.

6. The method according to claim 3 in Which the heat radiated toward thehearth at the portion thereof which receives the pasty mixture ofresidue and slag that gravitates down the sloping surface portion of thecharge on the hearth is sutficient to render that mixture suflicientlylimpid while on the hearth to cause it to flow to that heated portion ofthe chamber which is below the level of the adjacent portion of saidcharge, and said heated portion of said chamber is maintained at suchtemperature as will maintain said mixture in such limpid condition.

7. The method according to claim 3 in which the granular mixture fed tothe hearth also contains carbon reducible iron compound, coppersulphide, and precious metal, the heat radiated on the sloping surfaceportion of said mixture being suflicient also to react the carbon withsuch iron compound to form molten iron for dissolving copper sulphide ofsaid mixture, which iron also adheres to the residue of said mixture andgravitates with said residue and slag to the hearth; heating the pastymaterial so gravitating to the hearth while on the hearth to render itsufficiently liquid to cause the iron-copper sulphide solution toscavenge the residue of its precious metal values and to flow to thatheated portion of the chamber which is below the level of the edgeportion of the charge on the hearth, the temperature of said heatedportion of said chamber being sufiicient to maintain said materialflowing to its liquid, and upon removal of said material from saidchamber treating it to recover the precious metal values so scavenged.

References Cited in the file of this patent UNITED STATES PATENTS1,516,651 Tharaldsen Nov. 25, 1924 1,647,279 De Saulles Nov. 1, 19272,144,914 Debuch Jan. 24, 1939 2,377,478 Bowen Jan. 5, 1945 2,473,611Robson Jan. 21, 1949 2,482,445 Turner Sept. 20, 1949 2,598,743 Waring etal June 3, 1952 2,598,745 Handwerck et al. June 3, 1952

1. THE METHOD OF SMELTING ZINCIFEROUS MATERIAL CONTAINING ZINC OXIDE ANDSLAG FORMING CONSTITUENT WHICH COMPRISES FEEDING A GRANULAR MIXTURE OFSUCH MATERIAL AND REACTION CARBON TO A HEARTH IN AN OPERATIVELY CLOSEDCHAMBER AT SUCH RATE AS TO MAINTAIN CONTINUOUSLY ON SAID HEARTH DURINGTHE CONTINUANCE OF THE SMELTING OPERATION A MASS HAVING AN UPPER SURFACESLOPING DOWNWARD TO ADJACENT AN EDGE PORTION OF SAID HEARTH; RADIATINGHEAT DOWNWARD FROM ABOVE SAID MASS, WHILE SHIELDING THE PORTION THEREOFADJACENT SAID EDGE PORTION OF THE HEARTH FROM SUCH HEAT, TO REACT THEZINC OXIDE AND THE CARBON AT THE UPPER SURFACE PORTION OF THE UNSHIELDEDPART OF SAID MASS TO LIBERATE VAPOROUS ZINC FROM SUCH PORTION AND MELTSLAG FORMING CONSTITUENT OF SAID SURFACE PORTION TO CAUSE MOLTEN SLAGADMIXED WITH SOLID PARTICLES OF THE RESIDUE TO GRAVITATE DOWN SAIDSLOPING SURFACE TO THE SHIELDED PART OF SIAD MASS; REMOVING THE SLAG ANDRESIDUE MIXTURE FROM SAID HEARTH IN NON-LIQUID CONDITION, ANDDISCHARGING THE VAPOROUS ZINC FROM SAID CHAMBER.